A utility helicopter's ability to carry cargo externally is one of its most important features. Such utility helicopters are typically equipped to externally carry large, long or oddly shaped cargo provided that the cargo weight is within the lifting capacity of the helicopter. A significant advantage associated with this lifting capability is that a load may be picked up from or delivered to locations where access by other forms of transportation is difficult or impossible. Additionally, the systems do not require the helicopter to land to deliver or pick up the cargo.
Frequently, a helicopter will carry external cargo with a single or multi-point suspension system. In the case of a two point suspension system, the helicopter has two external cargo attachment points (suspension points) displaced longitudinally on the bottom of the aircraft, one on the center line forward of the aircraft center of gravity (c.g.) and one on the center line aft of the c.g. This arrangement stabilizes the cargo in yaw, thereby significantly reducing the cargo's ability to swing nose left and nose right.
A problem associated with a two point suspension system is that a failure of one suspension point results in all of the external cargo weight being supported by the other of the suspension points. The result of the total external cargo weight being supported at only one of the longitudinally displaced suspension points is the creation of large forces and moments about the aircraft c.g. (since the suspension point is on the bottom of the aircraft and some distance forward or aft of the aircraft c.g.). In some cases, these forces and moments can not be effectively resisted and the aircraft must release the load. The inability to resist these loads due to suspension point failure can be attributed to three factors: (a) the rapidity of the load application; (b) the magnitude of the load application; (c) a combination of both the rapidity and the magnitude of the load application. In the case of a rapid load application, the pilot does not have sufficient time to put in control inputs to eliminate the effects of the loads or reduce them to an acceptable level. Similarly, when the load application is of a large magnitude, the aircraft may not have sufficient inherent capability to reduce the effect of the loads to an acceptable level. The above problems are equally applicable to any multi-point suspension system upon failure of one or more of the suspension points or in the event of an uneven load distribution.
A solution to the above described failure problem is to release all external cargo suspension points in response to a failure of one of the suspension points. However, as noted above, the disruptive forces and moments may arise so quickly after a suspension point failure that the pilot or crew would be unable to release all suspension points before the aircraft response becomes severe. Referring to the example of FIG. 1, a forward suspension point failure in a two point suspension system is simulated for a CH-53E helicopter manufactured by Sikorsky Aircraft Division of United Technologies Corporation, Stratford, Conn. The following assumptions have been used in the example illustrated in FIG. 1: Aircraft weight is 36,000 lbs (16290 kg); load weight is 35,000 lbs (15840 kg); aircraft speed is 80 kts (600 m/s); and there is a 60/40 load distribution between the forward and aft suspension points, respectively. FIG. 1a shows the aircraft just prior to the failure, and FIGS. 1b through 1d show the aircraft at one second intervals after the failure. FIG. 2 is a graphical representation of the pitch rate response of the aircraft to the forward suspension point failure. As is seen in FIGS. 1 and 2, when carrying a large load using a two point suspension system, the aircraft experiences rapid and large transients upon failure of one of the suspension points (with no further action).
Because of the transient illustrated in FIGS. 1 and 2 above, an automatic cargo release system has been developed which monitors the cargo suspension points to determine if a load is attached to all suspension points. This monitoring is typically accomplished using switches which are located on hooks or attachment members which directly connect to the load. The switches are activated by the weight of the load. For example, in a two point suspension system, if one of the switches senses that no load is being applied at a suspension point for a threshold period of time, e.g., 0.5 seconds, then the load is released from both suspension points. One of the problems associated with an automatic release system of the type described herein above is that the aircraft has an ability to carry certain light loads on a single suspension point. The maximum weight which may be safely carried on one suspension point will depend on the helicopter's lifting ability. Another problem associated with the system described herein above is that for certain light loads, turbulence or aircraft maneuvers may cause an indication of no load on one of the suspension points for a time period greater than the threshold period even though the load is still securely attached. However, since the time period is exceeded, the load will still be released. A further problem with the above described system is that the location of the switches subjects them to becoming fouled with mud and debris or otherwise becoming damaged. Damage to one of the go/no go type switches may render the system inoperative or place the aircraft in an undesirable condition if a suspension point failure is not properly indicated, particularly when a heavy load is involved.